Development Of Design Criteria For Mechanical Roof Bolting In Underground Coal Mines

In spite of the widespread usage of mechanical roof bolts as a support system in the underground coal mines, the mechanisms by which roof bolts reinforce the mine roof are still not fully understood. The general practices of roof bolting system are largely based on some empirical rules, which tend to either underdesign or overdesign. In order to design the roof bolting system safely and economically, it is essential to understand the flexural behavior of the immediate roof. Based on the strata sequence, the strata in the immediate roof are divided into three types. The flexural behavior of the three strata types are investigated in terms of the following effects: roof span, horizontal stress, thickness and Young\u27s modulus of the lowest stratum. The reinforcement mechanism of suspension effect is analyzed based on beam-column theory. The equations of the maximum bending stress, deflection and transferred bolt load for the bolted strata are derived. In the analysis, the bolt load is assumed to be point load and the horizontal stress is uniformly applied to each stratum. The reinforcement mechanism of friction effect is also investigated. The major function of roof bolting in this case is to create the frictional resistance between the strata by the tensioning of roof bolts, thereby the individual layers are combined into one single thick layer. Based on the results of this research, the bolts should be equally spaced when the immediate roof is reinforced by the suspension effect. However the bolts should be spaced based on equal shear force concept when the immediate roof is reinforced by friction effect. An efficient computer program and nomographs are developed for the determination of proper bolting patterns and bolt tension. It is hoped that this development can lead to maximum safety with minimum cost for the design of roof bolting system in underground coal mines